Auto determine capabilities

lib/model_context_protocol/server.rb

@@ -22,7 +22,8 @@ def initialize(message, request, error_type: :internal_error, original_error: ni
 
     include Instrumentation
 
-    attr_accessor :name, :version, :tools, :prompts, :resources, :server_context, :configuration, :capabilities
+    attr_writer :capabilities
+    attr_accessor :name, :version, :tools, :prompts, :resources, :server_context, :configuration
 
     def initialize(
       name: "model_context_protocol",
@@ -33,7 +34,7 @@ def initialize(
       resource_templates: [],
       server_context: nil,
       configuration: nil,
-      capabilities: { prompts: {}, resources: {}, tools: {} }
+      capabilities: nil
     )
       @name = name
       @version = version
@@ -44,7 +45,6 @@ def initialize(
       @resource_index = index_resources_by_uri(resources)
       @server_context = server_context
       @configuration = ModelContextProtocol.configuration.merge(configuration)
-      @capabilities = capabilities
 
       @handlers = {
         Methods::RESOURCES_LIST => method(:list_resources),
@@ -64,6 +64,10 @@ def initialize(
       }
     end
 
+    def capabilities
+      @capabilities ||= determine_capabilities
+    end
+
     def handle(request)
       JsonRpcHandler.handle(request) do |method|
         handle_request(request, method)
@@ -154,6 +158,18 @@ def handle_request(request, method)
       }
     end
 
+    def determine_capabilities
+      defines_prompts = @prompts.any? || @handlers[Methods::PROMPTS_LIST] != method(:list_prompts)
+      defines_tools = @tools.any? || @handlers[Methods::TOOLS_LIST] != method(:list_tools)
+      defines_resources = @resources.any? || @handlers[Methods::RESOURCES_LIST] != method(:list_resources)
+      defines_resource_templates = @resource_templates.any? || @handlers[Methods::RESOURCES_TEMPLATES_LIST] != method(:list_resource_templates)
+      {
+        prompts: defines_prompts ? {} : nil,
+        resources: defines_resources || defines_resource_templates ? {} : nil,
+        tools: defines_tools ? {} : nil,
+      }.compact
+    end
+
     def server_info
       @server_info ||= {
         name:,

test/model_context_protocol/server_test.rb

@@ -739,5 +739,67 @@ class ServerTest < ActiveSupport::TestCase
       response = server.handle(request)
       assert_equal custom_version, response[:result][:protocolVersion]
     end
+
+    test "has tool capability only if tools or a tools_list_handler is defined" do
+      server_with_tools = Server.new(name: "test_server", tools: [@tool])
+
+      assert_includes server_with_tools.capabilities, :tools
+
+      server_with_handler = Server.new(name: "test_server")
+      server_with_handler.tools_list_handler do
+        [{ name: "test_tool", description: "Test tool" }]
+      end
+
+      assert_includes server_with_handler.capabilities, :tools
+
+      server_without_tools = Server.new(name: "test_server")
+
+      refute_includes server_without_tools.capabilities, :tools
+    end
+
+    test "has prompt capability only if prompts or a prompts_list_handler is defined" do
+      server_with_prompts = Server.new(name: "test_server", prompts: [@prompt])
+
+      assert_includes server_with_prompts.capabilities, :prompts
+
+      server_with_handler = Server.new(name: "test_server")
+      server_with_handler.prompts_list_handler do
+        [{ name: "test_prompt", description: "Test prompt" }]
+      end
+
+      assert_includes server_with_handler.capabilities, :prompts
+
+      server_without_prompts = Server.new(name: "test_server")
+
+      refute_includes server_without_prompts.capabilities, :prompts
+    end
+
+    test "has resources capability only if resources, template or custom handler is defined" do
+      server_with_resources = Server.new(name: "test_server", resources: [@resource])
+
+      assert_includes server_with_resources.capabilities, :resources
+
+      server_with_resource_template = Server.new(name: "test_server", resource_templates: [@resource_template])
+
+      assert_includes server_with_resource_template.capabilities, :resources
+
+      server_with_resources_list_handler = Server.new(name: "test_server")
+      server_with_resources_list_handler.resources_list_handler do
+        [{ uri: "test_resource", name: "Test resource", description: "Test resource" }]
+      end
+
+      assert_includes server_with_resources_list_handler.capabilities, :resources
+
+      server_with_resources_templates_list_handler = Server.new(name: "test_server")
+      server_with_resources_templates_list_handler.resources_templates_list_handler do
+        [{ uri_template: "test_resource/{id}", name: "Test resource", description: "Test resource" }]
+      end
+
+      assert_includes server_with_resources_templates_list_handler.capabilities, :resources
+
+      server_without_resources = Server.new(name: "test_server")
+
+      refute_includes server_without_resources.capabilities, :resources
+    end
   end
 end